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Anyone who has ever loved a grandmother or grandfather knows the nurturing role that grandparents can play. A study of indigenous people in Amazonia, who survive on food they hunt, forage or cultivate, quantifies the evolutionary benefit of that role. The results show that grandparents contribute a biologically significant amount of food calories to their extended families.

Proceedings of the Royal Society B published the results of the study of the Tsimane people of Bolivia, led by Paul Hooper, an anthropologist at Emory University.

“We quantified the net flow of calories among individuals in an environment where access to food is limited and depends on people generating it themselves,” Hooper says. “The results support the theory that grandparents are key to our relatively long childhood and long lifespan, which are a big part of what makes us human. Their efforts have likely been underwriting human society for hundreds of thousands of years.”

The study found that fathers and mothers contributed the most net calories to their nuclear family unit, followed by grandfathers and grandmothers, then uncles, aunts and children above the age of 12.